core,network: major per-object logging rework

      This changes log_unit_info() (and friends) to take a real Unit* object
insted of just a unit name as parameter. The call will now prefix all
logged messages with the unit name, thus allowing the unit name to be
dropped from the various passed romat strings, simplifying invocations
drastically, and unifying log output across messages. Also, UNIT= vs.
USER_UNIT= is now derived from the Manager object attached to the Unit
object, instead of getpid(). This has the benefit of correcting the
field for --test runs.

Also contains a couple of other logging improvements:

- Drops a couple of strerror() invocations in favour of using %m.

- Not only .mount units now warn if a symlinks exist for the mount
  point already, .automount units do that too, now.

- A few invocations of log_struct() that didn't actually pass any
  additional structured data have been replaced by simpler invocations
  of log_unit_info() and friends.

- For structured data a n